This glitch happened to me when transporting my horse across nether portals. After transporting my horse from the nether to the overworld there was about a 10 second delay where my horse seemed to be no where and then it spawned into the overworld. After moving my horse to an area where it could not reach the nether portal I went back into the nether and my horse was magically still in the nether. I put the horse back in its pen in the overworld without problem the second time. 10 minutes later I am using a different nether portal and my horse is found magically next to this separate nether portal that is in the nether. I transport it to this nether portal's overworld and it suffocates and dies in the blocks surrounding this overworld portal. I return to where I had left my horse about 10 minutes ago, in its pen, and it is now there. I was holding the saddle and leather that I got from the horse's death while looking at the same horse in the pen. I walked around the village where I kept my horse and the villagers that I had nametagged were duplicated and my singular black cat that I left leashed to a fence in my house was now three and the lead had also duplicated. Sorry this is so long, I felt it would be better if you had enough information to duplicate this scenario completely.

10 minutes after writing what is above: Glad I did more testing, the first part of this was all done in survival with cheats enabled via "Open To LAN", I closed the game completely and reopened and now all duplicated mobs are gone, all except for a villager that I nametagged, got duplicated and then became a zombie villager. The horse is still in its pen, I guess if there are duplicated mobs and you reopen the game, the unoriginal will despawn, also I'm guessing that the zombie villager didn't despawn because the game didn't see him as a duplicate anymore.